An objective of silicon photonics is to integrate different functionalities, including wavelength multiplexing and de-multiplexing, routing, optical emission, modulation, and detection on a silicon based platform, such as a silicon-on-insulator (SOI) based platform.
A compact, power-efficient and high-speed integrated silicon modulator is a key building block of silicon photonics. The carrier-depletion modulator is becoming a preferred solution for electro-optic modulation in silicon, because of its compatibility with complementary metal-oxide semiconductor (CMOS) process technology, its processing simplicity, and its high operation speed.
For example, a carrier-depletion based micro-ring modulator may comprise a micro-ring resonator with an integrated PN junction. The micro-ring can be coupled to a neighboring waveguide. The resonant wavelength may be modified by tuning the effective refractive index of the micro-ring waveguide. Tuning is obtained by reverse biasing the PN junction integrated with the micro-ring.
Due to its resonant nature, the micro-ring modulator has the advantages of compact size, low power consumption, and low driving voltage. However, a drawback of this structure is that optical modulation occurs only within a narrow wavelength range near the resonance wavelength. A typical optical bandwidth of effective modulation is less than 0.1 nm.
Further, a silicon waveguide is very sensitive to temperature variations. For example, a change in the ambient temperature of 1° C. results in a shift of the resonance wavelength of a silicon micro-ring modulator by about 0.1 nm. This makes the silicon micro-ring modulator very sensitive to any temperature fluctuation.
Therefore, a silicon micro-ring modulator generally needs to be integrated with a dynamic thermal stabilization system. Such dynamic thermal stabilization system may include a heater to change the temperature, a feedback circuit to control the heater power, and an optical power monitor that monitors the ring dynamics and that provides a feedback signal to the circuit controlling the heater power.
The 1.12 eV band gap of silicon makes it transparent in the telecom wavelength band around 1.55 μm. In order to enable monitoring of the optical power inside a silicon micro-ring modulator, several solutions have been proposed, such as: integration of a Germanium photodetector by eptiaxial growth; bonding of a III-V semiconductor based photodetector; or using a dedicated ion implantation step (typically Si+ implantation) to introduce lattice defects in the silicon waveguide in a predetermined area. These lattice defects may lead to defect-state mediated sub-bandgap absorption and result in the generation of a photocurrent. While the observed effect is generally much weaker than in case of direct absorption in a Ge or III-V photo-detector, it is an advantage of this approach that the functionality may be implemented relying exclusively on silicon carrier-depletion modulation without adding processing steps.
A silicon micro-ring modulator wherein defect-state mediated absorption is used for integrated power monitoring has been described by Hui Yu et al. in “Using carrier-depletion silicon modulators for optical power monitoring”, Optics Letters Vol. 37, No. 22, 2012. A micro-ring resonator with a lateral PN junction is described, wherein the PN junction is embedded in an SOI waveguide by ion implantation. After ion implantation, a rapid thermal annealing (RTA) may be performed at a temperature above 1000° C. to activate the dopants and to repair damage in the silicon lattice without causing much dopant redistribution. Still, there may remain some residual crystal defects that can mediate sub-bandgap absorption. It is shown that, after being heated to 1075° C., a reverse-biased PN diode still produces a substantial photocurrent, based on defect-mediated absorption in the ion-implanted SOI waveguides. This photocurrent is used as a power monitor feedback signal. This means that the widely utilized carrier-depletion modulator may also be used for optical power monitoring without any additional processing. For example, a small section of a PN-junction-embedded ring may be used to monitor the optical power inside the ring. It is suggested that the responsivity of both the modulator and the monitor may be enhanced by using an interdigitated PN junction instead of a lateral PN junction, to enlarge the overlap between the optical mode and the carrier-depletion region, or by reducing the RTA temperature to increase the crystal defect density.